This invention relates to an analogue-to-digital conversion apparatus.
Known said apparatus includes an analogue-to-digital converter and a system for controlling the analogue input to the converter. The converter may be said to divide each instantaneous input value according to the expression V/2.sup.n where V is the maximum input value and n is the number of digits of the converter necessary to express the input value in binary terms. The resolution may be said to be based on the divisor 2.sup.n. The value V/2.sup.n defines the basic resolution of the apparatus and may be regarded as the smallest fraction of V which the converter can identify or the smallest change of the input which the converter can resolve. Thus if the input value is said maximum, the converter can identify 2.sup.n steps. If a smaller input value is applied, the number of steps is reduced to 2.sup.n-q, where q=the number of digits, at the most significant end of the converter, remaining "0". Therefore, if the input is smaller, the effective resolution by which this input may be described is reduced. If it is then desired to improve the resolution, this can be done in a known said system by amplifying the input value to the extent that more of the digits of the converter are used. The resolution is then improved because the input value is divided by a larger number than would be otherwise the case.
However, a difficulty arises if the input value varies over a range the upper values of which are close to the numerical range of the converter, i.e. close to the condition in which all digits of the converter become binary "1". In such a case the resolution of the lower values of the input range cannot be improved by amplification because the correspondingly amplified upper values of the input range would then lie outside the numerical range of the converter, and the converter becomes "over-ranged". The difficulty is particularly acute if the input values vary randomly or in an unpredictable manner. An example of this is found in cases where the input is derived from a varying light signal. It is an object of this invention to overcome or reduce this difficulty.